(1) Field of the Invention
The present invention relates to a radio frequency power amplifier used for transmission in mobile communication such as a mobile phone.
(2) Description of the Related Art
High efficiency (power saving) of transmission power amplifiers which consume a large amount of power as well as the miniaturization of batteries have been important in order to achieve mobile phones which are small, lightweight and are able to perform communication for extended periods of time. The power amplifiers used for the transmissions carried out by the mobile phones are called power amplifier (PA) modules. GaAs radio frequency transistors, which are excellent in radio frequency characteristics and power conversion efficiency, are mainly used as such PA modules. The GaAs radio frequency transistors include field-effect transistors (hereinafter referred to as “FETs”) and hetero-bipolar transistors (hereinafter referred to as “HBTs”).
In a CDMA scheme, such as Wideband-Code Division Multiple Access (W-CDMA), a method is used in which radio frequency power output from antennas of mobile terminals are adjusted according to the distance to a base station or surrounding environment so that radio frequency power arriving at the base station are approximately equal to each other. Generally, when the distance to the base station is far, radio frequency power output from an antenna is high, and when the distance to the base station is near, the radio frequency power output from the antenna is low. The radio frequency power is output from the antenna through adjustment of the output of the PA module. The output from the antenna is often relatively low; and thus, achieving high efficiency of the PA module in the low output state is extremely important for reducing power consumption.
FIG. 1 is a block diagram of a PA module disclosed in Patent Reference 1 (U.S. Pat. No. 7,248,111). Hereinafter, a conventional PA module is described with reference to FIG. 1. In the following description, like numerals are used for like elements.
As an example, the PA module is designed such that a main circuit operates when the radio frequency output from an output terminal 122 is greater than 15 dBm, and a subcircuit operates when the radio frequency output from the output terminal 122 is equal to or less than 15 dBm.
First, operations of the main circuit are described. The radio frequency power input to an input terminal 121 is input to an earlier stage HBT 101 via an input matching circuit 111. The radio frequency power amplified by the earlier stage HBT 101 is input to a subsequent stage HBT 102 via an inter-stage matching circuit 112. The radio frequency power amplified by the subsequent stage HBT 102 is output from an output terminal 122 via an output matching circuit 113.
Next, operations of the subcircuit are described. The radio frequency power input to the input terminal 121 is input to a sub HBT 103 via an input matching circuit 114. The radio frequency power amplified by the sub HBT 103 is output from the output terminal 122 via a matching circuit 115, a switch 105, and the output matching circuit 113.
However, the conventional PA module in FIG. 1 has a problem in that ZA at the time of operation of the main circuit and ZB at the time of operation of the subcircuit cannot be designed independently.
In the block diagram of the conventional PA module in FIG. 1, let the point at a collector terminal side of the subsequent stage HBT 102 be A, the point at an output node side of the switch 105 be B, the point at an input side of the output matching circuit be C, the intersection point of the subcircuit with the main circuit be D, the impedance looking into the point D from the point A be ZA, the impedance looking into the point D from the point B be ZB, and the impedance looking into the output terminal 122 from the point C be Zc. When the main circuit operates, the switch 105 is turned off; and thus, the impedance looking into the point B from the point D is open (high impedance). Further, when the subcircuit operates, the subsequent stage HBT 102 is turned off; and thus, the impedance looking into the point A from the point D is open. Therefore, ZA at the time of operation of the main circuit and ZB at the time of operation of the subcircuit each needs to be equal to ZC to match the output matching circuit 113. As a result, ZA and ZB cannot be designed independently.
The following describes a specific example of a disadvantage which can occur. In the configuration of the conventional PA module of FIG. 1, in the operation of the main circuit, adjacent channel leakage power ratio (hereinafter, simply referred to as ACPR) of −40 dBc or less and efficiency of power conversion (hereinafter, simply referred to as efficiency) of 40% or more are required under the conditions of frequency of 1920 MHz, power voltage of 3.5 V, and output power of 28 dBm. In order to meet such characteristics, it is preferable to set ZA to 4Ω. As a result of evaluation of the PA module shown in FIG. 1 under the load condition, characteristics of ACPR of −42 dBc and efficiency of 42% were obtained in the frequency of 1920 MHz and the output power of 27 dBm.
In the operation of the subcircuit, ACPR of −40 dBc or less and efficiency of 23% or more are required under the conditions of the frequency of 1920 MHz, the power voltage of 3.5 V, and the output power of 16 dBm. In order to meet such characteristics, it is preferable to set, to 50Ω, the impedance Zsub looking into the matching circuit 115 side from the collector terminal of the sub HBT 103. At this time, the switch 105 is in its on state, and the on-resistance is 2Ω. ZB is 4Ω, being equal to ZA.
FIG. 2 is a graph showing relationship between ZB and switch loss in the conventional PA module. For example, where the resistance of the switch 105 is 2Ω, loss at the switch 105 is 1.75 dB when Zc is 4Ω. When the resistance of the switch 105 is 2Ω, it is also possible to reduce the loss to 0.8 dB by setting ZB to 10 Ω.
As shown in FIG. 2, when the on-resistance of the switch 105 is 2Ω, and ZB is 4Ω, loss of radio frequency power at the switch 105 is 1.75 dB. As a result of the evaluation of the subcircuit shown in FIG. 1 with the above condition, ACPR of −42 dBc and efficiency of 21.5% were obtained under the conditions of the frequency of 1920 MHz and the output power of 16 dBm. This does not meet required characteristics.
Reduction in loss of the switch 105 is required to improve the efficiency of the subcircuit; however, if the value of ZB increases to be greater than 4Ω to achieve the reduction, the value of ZA also increases. As a result, the characteristics required for the main circuit are not met.
On the other hand, if the gate width of a FET used as the switch 105 is increased to reduce the loss of the switch 105, the on-resistance of the switch is 1Ω when the gate width is 2 mm. Thus, the loss at the switch 105 is 0.8 dB at a rough estimate. As a result of the evaluation of the subcircuit shown in FIG. 1 with this condition, ACPR of −42 dBc and efficiency of 23.5% are obtained when the frequency is 1920 MHz and the output power is 16 dBm. This meets the required characteristics. However, by increasing the gate width of the FET from 1 mm to 2 mm, the chip size also increases. This results in increase in the cost of the PA module.